The present invention concerns a method for hermetically encapsulating microsystems in situ. At least one microsystem mounted on a substrate is encapsulated under a metal capsule made in situ. xe2x80x9cMountedxe2x80x9d means either placing the microsystem, made beforehand, on the substrate, or making the microsystem in situ on the substrate. Preferably, several microsystems of micrometric dimensions are manufactured together on the same substrate. The encapsulation enclosing the microsystem must be sealed hermetically and leave said microsystem free of movement inside the capsule.
xe2x80x9cMicrosystemsxe2x80x9d means three-dimensional structures, i.e. microoptoelectromechanical devices (MOEMS) or microelectromechanical devices (MEMS) such as reed contactors, accelerometers, micromotors, sensors of micrometric size, which need to be left free to move after encapsulation. The construction of said microsystems can be made on an insulating substrate or on a substrate comprising integrated circuits which have been made beforehand. In this latter case, it is possible to use the metal contact pads of the integrated circuit to begin depositing the metal layers which will form part of the microsystem and to allow it to be electrically connected to said circuit.
Swiss Patent No. 688213, by the same Applicant, discloses a reed contactor or contactor with strips of micrometric size and the manufacturing method thereof. The contactor comprises metal strips at a distance from each other in the rest state which are made by electrolytic means in several steps and are attached to a base plane. The strips are formed of an iron and nickel alloy, deposited by an electrolytic method. This alloy has the property of being ferromagnetic so that the strips are able to be put in contact with each other when a magnetic field passing through them creates a force of attraction therebetween. This contactor is encapsulated under a hollow cover which is fixed for example using an epoxy adhesive material onto the base plane. The latter may be a glass substrate or an insulating layer obtained by oxidising the surface of a silicon substrate. The cover is formed of a glass plate in which cavities are formed by chemical etching. This plate allows each contactor to be enclosed in each of the etched cavities. The plate may be bonded onto the base plane or soldered by an eutectic or anodic solder. In a final operation, the multitude of contactors thereby made and sealed are separated by cutting or dicing operation.
In this type of embodiment, it is necessary to machine the glass plate separately from the substrate on which the contactors are manufactured. This constitutes a drawback. Moreover, the plate has to be bonded precisely onto the base plane using an epoxy adhesive material. The sealing is not hermetic over the long term, since the epoxy resin absorbs water and degasses substances capable of disturbing the operation of the contactor. In other embodiments, an heat treatment for encapsulating the contactor can be destructive.
In Swiss Patent No. 688213 it will also be noted that during contact resistance measurements between the metal strips, prior to encapsulation of the contactors, the contact resistance average of all the contactors made on a same substrate was centred around 10 ohms. After said encapsulation, this contact resistance average was measured rising to 10 to 60 ohms.
European Patent No. 0 302 165 discloses a sheet of tin which is formed by stamping to act as the metal dome for an integrated circuit. This stamped sheet is then bonded onto a base plate where the integrated circuit is placed so as to close said circuit under the dome. The whole assembly is subsequently coated with a layer of polyethylene. The adhesive material, as explained above, can cause contamination of the microsystem. Consequently it does not allow hermetic encapsulation to be guaranteed. It is also not possible to design the dome in situ by stamping. Moreover, making these stamped sheets, which have to be individually placed on each microsystem, complicates the encapsulation of several microsystems mounted on a same substrate.
In the field of combined micromechanical and electronic devices, the use of sacrificial layers is already known. One can cite the case in which one wishes for example to make a metal bridge between an integrated circuit and a sensor. On the other hand in the case of making an hermetic metal encapsulation for microsystems, the use of sacrificial layers is not known.
U.S. Pat. No. 5,798,283 discloses a method for manufacturing at least one microelectronicmechanical device with an electronic circuit. A cavity is etched in the substrate for example made of silicon in order to house therein the micromechanical device. The latter is constructed using different layers of polysilicon in order to obtain elements able to be free of movement. The device has to be protected using layers of silicon oxide or nitride so that the subsequent steps for making the integrated circuit can be performed. This protection of the micromechanical device is necessary to protect it against dopant diffusion temperature (boron, phosphorous for example) which can be higher than 700xc2x0 C. Such temperature can partly destroy the elements of said micromechanical device designed with certain metals with a low melting point. Such protecting layers also allow to avoid doping said elements if polysilicon is involved to be avoided.
Once the integrated circuit operations are finished, two openings arranged in a protective layer disposed above layers of SiO2 or Si3N4 allow said layers of SiO2 or Si3N4 to be partly removed by chemical etching. That allows thus to release the micromechanical device and to leave it free of movement. During such removal, precautions must be taken to avoid too great a lateral etching, because the integrated circuit is constructed beside the micromechanical device.
Instead of making two openings in the protective layer, it might have been envisaged to use only one layer of porous polysilicon in order to remove the layers of SiO2 or Si3N4 by chemical etching, in particular using fluorohydric acid, through the polysilicon, and then to rinse with deionised water.
Several drawbacks of said method from this document can be cited. Firstly, the encapsulation is made using non-metallic layers. Moreover, a cavity has to be arranged beforehand in the substrate to house therein the microsystem by etching techniques similar to those used in the microelectronic field. The microsystem has also to be protected while the corresponding integrated circuit is being made with layers which can withstand high temperatures. Consequently, there is no question of depositing metal layers in particular by electrolytic means on said micromechanical device to create an hermetic metal encapsulation.
European Patent No. 0 435 530 discloses an electronic system hermetically sealed by metal layers one of which is deposited by electrolytic means. The electronic system is an association of different integrated circuits, with high density interconnection (HDI). These circuits are housed and bonded using polymers in a cavity micro-machined in a glass or ceramic substrate. A first metal layer, in particular made of chromium or titanium, is sputtered onto a dielectric layer which overhangs the interconnections made for the different circuits. This first layer allows to coat the entire structure and to come into contact with the surface of the substrate. Subsequently, a second metal layer is deposited by electrolytic means above the first layer in order to create a thicker protective layer against various contaminating elements able to disturb the circuits.
European Patent No. 0 435 530 provides no teaching for making an encapsulation for microsystems, such as reed type contactors. One drawback is that the polymers used to bond the circuits, produce gases, i.e. degas. That thus creates defects which will be noticeable as regards the proper operation of the contactor. Moreover, it is to be noted that creating a metal capsule via a sacrificial metal layer removed after deposition of a subsequent metal layer forming the capsule, is not envisaged.
One object of said invention is to provide an hermetic encapsulation in situ for microsystems which overcomes the drawbacks of the aforecited prior art.
Another object of the present invention is to be able to make a metal capsule via electrodeposition of metal layers for encapsulating microsystems at temperatures lower than 350xc2x0 C. maximum. This overcomes the drawbacks of methods of prior art wherein, in particular, the diffusion of phosphorus or boron for integrated circuits occurs at temperatures exceeding 700xc2x0 C. and able even to reach 1300xc2x0 C.
Another object of the invention is to avoid a large dispersion of contact resistance values after hermetic encapsulation. The microsystem can be a contactor which has to be in an inert or reducing atmosphere.
These objects, in addition to others are achieved as a result of the method for hermetically encapsulating microsystems in situ wherein, in a first phase, several microsystems are mounted on a common substrate, said microsystems being surrounded by a metal adhesion layer deposited on the substrate, the method being characterised in that, in a second phase, in a common deposition operation a first metal layer is deposited on each microsystem and on an annular zone of the adhesion layer surrounding each microsystem so as to completely cover each microsystem by overlap, in that a second metal layer is deposited by electrolytic means on the first layer and on the adhesion layer so as to cover the first layer over most of its surface leaving at least one passage per microsystem in the second layer to provide access to the first layer, the metal of the first layer being different from the metals of the adhesion layer, the second layer and the microsystem, in that the first layer is removed by selective chemical etching through each passage arranged in the second layer, and in that each passage in the second layer is closed or sealed to obtain metal capsules hermetically enclosing each microsystem.
One advantage of the method of the invention consists in making an hermetic metal encapsulation using means which allow simultaneously processing of substrates on which several microstructures have been mounted. The microstructures are made for example in situ onto the substrate. However, they can be made too beforehand and placed after onto the substrate.
Another advantage of the method of the invention lies in the fact that the metal capsule made on the substrate and enclosing the microsystem is held without the use of adhesive materials. Said adhesive materials may contain polymers capable of degassing contaminating elements inside the metal capsule, liable to disturb the microsystem.
The creation of a metal encapsulation for microsystems using depositions of metal layers has thus been envisaged. One of metal layers acts as a sacrificial layer. Moreover, at least the final metal layer is deposited by electrolytic means on a metal adhesion layer which adheres well to the insulating surface of the substrate.
In order to make this capsule, a first metal layer, called the sacrificial layer is deposited, preferably by electrolytic means, onto the whole of the microsystems and onto annular zones of the adhesion layer surrounding each microsystem. The first layer allows to completely cover each microsystem by overlapping. After this first metal layer has been deposited, the covered microsystems have a dome shaped appearance. A second metal layer is then deposited by electrolytic means onto the first layer, said second layer having passages providing access to the first layer.
The first metal layer is formed of a different metal to the metals forming the second layer, the adhesion layer and also the microsystem. This first layer is able to act as the sacrificial layer to be removed selectively by chemical etching through at least one passage made in the second metal layer in order to make the metal capsule. In a final encapsulation step, it is necessary to close or seal the passage or passages made in the second layer in order to hermetically close the capsule while keeping inside the capsule the microsystem in an inert or reducing atmosphere.
xe2x80x9cMetalxe2x80x9d also includes all the metal alloys depending on a particular metal.
This electrodeposition technique allows high quality encapsulation of microsystems at a low cost.
Another advantage of the method of the invention is that it avoids having to protect the microsystem for the subsequent manufacture of the integrated circuit arranged next to it as described in U.S. Pat. No. 5,798,283. In the case for example of a microcontactor, these encapsulation steps even occur at the ambient temperature.
In a preliminary phase of the method, one may for example form on a substrate, of which at least one surface is insulating, conductive strips for the electric connection of the microsystem with the exterior. An insulation of the median portion of the strips is then made. Moreover, a surface metallisation connects one end of the strips and also passes above the insulation of the strips. Also in this first phase of the method, the microsystem to be encapsulated is mounted on the substrate. In a second phase, the metal capsule is formed with the closing of its orifices. The substrate may be cut subsequently to obtain a multitude of encapsulated microsystems.
The invention will be better understood with reference to the drawings showing non limiting embodiment examples of the method of the invention in which: